Pakistan's special forces on Tuesday stormed a counter-terrorism center in a remote northwestern district to free several security officials who were taken hostage earlier this week by a group of detained Pakistani Taliban militants, security officials said.

The operation came after the detainees, who were held for years at the center in Bannu, in Khyber Pakhtunkhwa province, overpowered their guards on Sunday, seized their weapons and took them hostage. On Monday, one officer at the center was reported killed by the hostage-takers.

Read more: Pakistani Taliban overpower guards, seize police center

Pakistani officials tried to negotiate with the hostage-takers but after more than 40 hours of failed efforts, special forces deployed to the area stormed the compound, security and intelligence officials said. They did not elaborate.

By Tuesday afternoon, thick black smoke billowed into the sky from inside the compound, after two explosions were heard. Intermittent gunshots were reverberating across the area, the officials said. They spoke to The Associated Press on condition of anonymity to discuss the ongoing situation.

It was not immediately clear what had happened to the hostages or the Taliban fighters. No military or government spokesmen were immediately available for comment.

Earlier, officials said there were about 30 Taliban fighters involved in the takeover of the center. The hostage-takers had demanded a safe passage to the former strongholds of the militant group.

Read more: Afghan forces shell border town, killing 6: Pakistani army

The brazen taking over the center on Sunday was a reflection of the government’s inability to exercise control over the remote region along the border with Afghanistan.

The Pakistani Taliban are also known as Tehreek-e-Taliban Pakistan or TTP. They are separate but allied with the Afghan Taliban who seized power in neighboring Afghanistan last year as U.S. and NATO troops were in the final weeks of their pullout from the country after 20 years of war.